The present invention relates to a process for fixing a fitting on a sintered metallic filtering element and to the product thus obtained.
Metallic filtering elements are generally produced by sintering a metal powder, for example of 316 type stainless steel according to the AISI nomenclature, by adjusting the porosity to obtain the desired filtering effect while having the slightest possible loss of charge for a given flow rate; they are generally in the form of a cylindrical tube which is closed at one end and comprises, at the other end, a fitting producing the connection to a load-bearing structure such as a plate perforated by orifices. This unit is frequently called a "candle filter". According to the prior art, the fitting is either welded to the porous tube or is metallurgically fixed thereon during sintering.
The first solution has the following drawbacks:
welding leads to the presence of a region which is affected by heat and is generally more fragile PA1 the weld obtained has to be perfectly sound, the presence of fissures generally leading to poor filtration and, in certain cases, to mechanical breakages, particularly if the weld is located close to the fixing, that is to say in a region where the bending moment (due to vibrations for example) is high. PA1 the poorly controlled shrinkage during sintering can lead to local detachment or fissuring in the region of the connecting interface PA1 the length of the candle cannot be adjusted after sintering and is therefore subject to variations PA1 the technique employed prevents compression through the interior of the tube PA1 the fitting has to have dimensions and a shape close to that of the porous tube. PA1 on the one hand, producing an externally cylindrical, porous sintered tube of which the connecting end has excess thickness delimited by a circular internal truncated cone and having a half angle at the vertex .alpha.=0 PA1 on the other hand, producing a tubular connecting member which generally has an external thread, is equipped with a truncated cone-shaped end and has the same half angle at the vertex .alpha. as the sintered tube PA1 force fitting the tubular connecting member in the tube so that the mean normal stress between the two aforementioned coupled elements is greater than twice the mean normal stress imposed by the mounting of the candle on its support. PA1 on the one hand, producing an externally cylindrical porous sintered tube of which the connecting end has an excess thickness delimited by a circular internal truncated cone and having a half angle at the vertex .alpha. of between 4.degree. and 7.degree. over a height (h) of between 20 and 80 mm PA1 on the other hand, producing a tubular connecting member which is generally externally threaded, is equipped with a truncated cone-shaped end and has the same half angle at the vertex as the sintered tube over a height h' which is smaller than h, of which the small diameter (d) is greater than the orifice of the tube (t) and of which the large diameter (D) is smaller than the internal diameter of the tube (Ti). PA1 force fitting the tubular connecting member in the tube so that the mean normal stress between the two aforementioned coupled elements is greater than twice the mean normal stress imposed by the mounting of the candle on its support (and remains greater than 2 MPa).
In the second solution, a solid metallic fitting is positioned at one end of the cold compression mould (for example by isostatic pressure) before introduction of the powder into the annular space delimited by a central mandrel and the external flexible casing of plastics material. After compression, the unit is sintered, producing a metallurgical connection between the fitting and the tube.
However, this method has the following drawbacks: